Not applicable.
Not applicable.
This invention relates generally to optical systems and more particularly to optical systems for use with inspection of printed circuit boards.
As is known in the art, many systems for inspecting assembled printed circuit boards (PCBs) typically use black-and-white charge coupled device (CCD) cameras. With only black-and-white images, the detection of certain classes of component becomes impossible or highly unreliable at best, such as the detection of a black component on a dark green PCB. Such difficulties result in systems having relatively high false detection rates, with undesirable results for the customer. For example, a PCB with 1000 components is not unusual, but a false rate of only {fraction (1/1000)} means that 63% of PCBs will have at least one inspection error.
Neither PCBs nor components are fabricated with machine vision inspection in mind. Thus, the variety of PCB and component materials and surface appearance pose additional difficulties for black-and-white imaging systems.
To further complicate the inspection process, there is a large variation in the location and shape of component orientation marks. This can make the orientation marks difficult to detect without some optimized lighting. The variation in appearance even extends to fiducials, which provide the coordinate system for the PCB. So-called Hot Air Solder Leveled (HASL) fiducials are common and have an irregular shaped dome-like appearance which makes them difficult to find accurately with the lighting in many inspection systems. This in turn makes accurate position measurements of the components on the PCB difficult.
It is a known principle in optics that the angle of illumination of an object influences both the intensity and spectral distribution of the reflected light. If one could optically model the surface of an object, it would be possible to predict the spectral distribution and intensity of light reflected by the object. The surfaces of real objects having irregular shapes, however, are so complicated that modeling the surfaces is relatively difficult and in some cases may not be possible.
To distinguish between two objects or between two different surfaces of the same object, some inspection systems use color lights for inspection. Such systems typically include a bank of different color light sources. For each object type, a particular color light source projects a light that maximizes the contrast of a surface of the object and another surface. For example, in printed circuit board inspection systems, a particular color light source projects a light that maximizes the contrast of a surface of a component and a surface of a printed circuit board on which the component is disposed. One problem with such systems, however, is that they require several light sources each of which projects a different color light. Moreover, a system user must know which color light to used for each different type of object or component.
It would, therefore, be desirable to provide an optical system which improves the ability to detect PCB components, fiducials and other PCB features with a relatively high degree of accuracy. It would also be desirable to provide a relatively simple optical system to distinguish between two objects or between two different surfaces of the same object.
In accordance with the present invention, an optical system includes an imaging system and a lighting system. The imaging system utilizes spectral reflectivity to discriminate between electronic components (components) to be inspected and a surface of the printed circuit board (PCB) on which the components have been placed. The lighting system includes a first light source disposed to project diffuse light along or coaxial with an optic axis of the imaging system and a second light source or sources disposed to project diffuse light from a position which is to the side of the object to be inspected.
With this particular arrangement, an imaging system which provides good discrimination between an object to be inspected (e.g. an electrical component) and a second object (e.g. a surface of a printed circuit board in which the object is disposed) is provided. The purpose of the diffuse coaxial lighting is to illuminate metallic surfaces on the PCB (including but not limited to HASL fiducials, pads of component footprints, component leads, etc.) so that specular highlights are minimized. For this purpose, xe2x80x9cdiffusexe2x80x9d means that the light rays from the coaxial lighting be incident on these metallic surfaces at angles of zero to at least 10 degrees with respect to the optic axis. This lighting, although very effective for its stated purpose, provides little discrimination between the components and the surface of the PCB on which the components are to be disposed (e.g., PCB material FR4 with solder mask). This is due to the optical reflection characteristics of the PCB surface.
The PCB reflection/optical characteristics of the PCB surface, mainly because of the solder mask characteristics, will vary from highly reflective (specular) to matte. The closer the reflection is to being specular, the less color information it carries, meaning that colored PCB surfaces will appear desaturated or neutral rather than colored. Thus, the system also includes diffuse lighting from the side, at a central angle typically in the range of about 30 to about 60 degrees with respect to the PCB surface and symmetrically placed about the optic axis. The illumination provided by the side lighting helps the camera distinguish the components from the PCB surface.
Because of the high reflectivity of the metallic surfaces on the PCB, the intensity of the coaxial illumination is selected to be lower than the intensity of the side illumination. Also, the spectral distribution of the coaxial light is selected to be significantly different from the spectral distribution of the side lighting. This can be achieved in several ways. For example, lights of different correlated color temperatures, or lights of similar correlated color temperatures combined with different spectral bandpass filters. This allows discrimination between footprint pads (reflective) and solder paste, which may or may not fully cover the pads, by measuring the spectral characteristic of the reflected light. Here xe2x80x9ccorrelated color temperaturexe2x80x9d refers to visual appearance equivalent to a blackbody radiator at a given color temperature, as defined by CIE.
Having two types of lighting also provides flexibility for adjustment of side lighting angle and relative intensity for better detection of orientation marks on components.
The optical system lensing is chosen to be telecentric to avoid perspective effects and minimize the change in magnification with the height of the object viewed. This is very important for dimensional measurement on the assembled boards being inspected, where components can have substantially different heights.
Thus, advantageous features of the optical system of the present invention include: (1) spectral discrimination through the use of a combination of coaxial and side lighting; (2) recognition that diffuse side lighting provides good spectral discrimination between components and a PCB surface; (3) recognition that coaxial lighting and side lighting with a significantly different spectral distribution provides significant spectral discrimination between specular reflective surfaces (e.g., footprint pads) and diffusing surfaces (e.g., solder paste); (4) use of coaxial lighting intensity which is lower than the side lighting intensity.
In accordance with a further aspect of the present invention, an optical system includes an imaging system and a lighting system disposed about the imaging system. The lighting system includes a first light for providing diffuse illumination to a first region along an optical path approximately coincident with an optic axis of the imaging system and one or more second lights, each of the one or more second lights for providing diffuse illumination to the first region along a path which is at an angle with respect to the optical path of the color imaging system.
The imaging system can be provided as a multi-spectral band imaging system which includes a CCD color camera having a telecentric lens coupled thereto. The telecentric lens avoids perspective effects and the lighting provides not only spectral discrimination but contrast between metallic surfaces such as fiducials and the PCB surface, while minimizing specular highlights.
It should be appreciated that the light sources provided as part of the illumination system do not have to be different spectral distributions. That is, the spectral difference can come from the lights themselves or from identical lights plus a spectral filter. Thus, the light could be passed through different spectral bandpass filters. In one embodiment, a continuous fluorescent light is used. Fluorescent lights come in different correlated color temperatures. An alternative approach is to use two xenon strobe lights. The strobe lights would preferably be the same color, so spectral filters would be used.
For non-strobe lighting, the camera exposure typically needs to be tens of milliseconds. This would cause image blurring in a moving camera. Because of this, the motion system carrying the camera has to come to a stop and settle before the image can be acquired. The benefit of the strobe light approach is that because the duration of the strobe light pulse (typically tens of microseconds) is so short that the camera can be in rapid motion while the image is taken, without image blurring.
When illuminated from the top only, the surface of the printed circuit boards appears desaturated or neutral gray, thus the board does not look very different from the components on top of it. It has been recognized in accordance with the present invention that side lighting brings out the spectral reflection characteristic of the board. Recognizing that this difference results from use of both top and side illumination allows the system to spectrally discriminate between board and component.